Fluoroelastomer coatings have a wide variety of industrial uses. Such coatings are commonly used to provide corrosion protection for metals in corrosive services, to reduce friction between metals in sliding services, or to act as release surfaces in services in which such surfaces are desirable. Fluoroelastomers typically used as coatings include perfluoroalkoxy (PFA). In order for these materials to provide the desired properties, they should be applied to the metal substrate in such a way that they form a continuous, pinhole-free coating.
Fluoroelastomer coatings are difficult to apply to metal surfaces because they are generally inert and therefore, cannot be readily bonded chemically to metal surfaces. Moreover, fluoroelastomers have coefficients of thermal expansion several times larger than the metal substrates to which they are applied. As a result, fluoroelastomer coatings which are exposed to significant changes in temperature tend to slough off the metal substrates. Both of these problems may be overcome by mechanically bonding the fluoroelastomer to a porous metal surface, or tie coat, which can be bonded to the metal substrate. The tie coat contains numerous pores and cracks. The mechanical bond between the fluoroelastomer coating and the metal tie coat is formed when molten fluoroelastomer is allowed to flow into the pores and cracks of the tie coat and solidify.
Several techniques are available to prepare the tie coat. One method is to spray molten metal onto the surface of the substrate using a thermal or plasma spray gun. The tie coat, which cools rapidly, becomes porous as it solidifies. Another method is to electroplate a metal onto the substrate and then make it porous by reversing the polarities of the electrodes in the electroplating means. This electroplating technique is described in U.S. Pat. No. 3,591,468 to Nishio et al. The tie coat may be primed to improve fluoroelastomer adhesion. Typical primers include highly fluid fluoroelastomer compounds such as fluoroethylenepropylene (FEP) or inorganic primers such as CaSiO.sub.3 or Cr.sup.++.
The fluoroelastomer coating is normally applied to the tie coat by a series of deposition steps. Each deposition step comprises heating the tie coat to a temperature which will melt the fluoroelastomer followed by electrostatically depositing the fluoroelastomer in the form of a powder to a thickness of between approximately 0.5 mil and approximately 1 mil. A plurality of deposition steps is used to produce coatings thicker than approximately 1 mil. If this method is used to deposit coatings thicker than approximately 1 mil with a single deposition step, the coating will skin over, resulting in an aerated, porous, low density coating. Such a coating might not provide the desired protective properties. Skinning occurs when a coating which has been applied as a low density powder fuses on the surface and traps air within the rest of the coating. In addition, a coating which has been applied to a thickness greater than approximately 1 mil with a single deposition step may tend to fall from the surface of the tie coat when the coated substrate is moved.
Using this sequential deposition method to produce fluoroelastomer coatings thicker than approximately 1 mil is very time consuming and labor intensive. Labor costs can account for up to 80% of the total cost of a fluoroelastomer coating applied by this method.
Accordingly, there has been a continuous effort in this field of art to develop a quicker and less labor intensive means for producing a continuous fluoroelastomer coating of the desired thickness.